As a variety of electronic devices are miniaturized and become light-weight, a photo-sensitive solder resist which can form minute opening patterns is being used in a printed circuit board, a semiconductor package board, a flexible circuit board, etc.
The semiconductor package product is a complex material consisting of non-conductors such as epoxy molding or solder resist, semiconductors such as chip die and conductors such as board circuit patterns, and several processes accompanying severe heat shock conditions should be performed in order to prepare it. However, the problems of dimensional instability and a warpage phenomena appear due to different coefficients of thermal expansion (CTE) of the non-conductor, semi-conductor, and conductor. Such phenomena may cause the position mismatch between the chip and board when the chip die and semiconductor board are connected by a solder ball or a gold wire, and a crack and rupture of a product due to the shear stress, which influences the lifetime of the product. As boards are recently becoming thinner, such problems of dimensional instability and the warpage phenomena become more serious. In order to solve the above, materials have been developed toward minimization of the CTE mismatch between the materials. Also, it is consistently needed to develop a solder resist having a lower coefficient of thermal expansion.
The previously known dry film-type solder resist has a coefficient of thermal expansion whose α1 (thermal expansion coefficient before Tg) is 45 to 70 ppm and α2 (thermal expansion coefficient after Tg) is 140 to 170 ppm.
As for the core among the current board materials, it has been reported that materials having a thermal expansion coefficient of 10 ppm or lower or even 5 ppm or lower were developed. However, the development of a material for a solder resist that can be used along with the core has not yet been reported.
Attempts have been made to lower the thermal expansion coefficient of the solder resist by increasing the content of a filler used. However, if the content of the filler is increased higher than a certain level, poor coating may be caused due to filler cohesion, and elongation may be reduced after coating and before curing to result in the problem of deteriorated workability.
The characteristics of a developing property, high resolution, an insulating property, an adhesive property, resistance to soldering heat, tolerance to gold plating, and the like are generally required of the solder resist. Particularly, for example, a crack-resistant property against a temperature cycle test (TCT) from −65° C. to 150° C., or a highly accelerated stress test (HAST) property to fine wiring, in addition to said characteristics, are required of the solder resist for a semiconductor package board.
Recently, a dry film-type solder resist superior in uniformity of film thickness, surface smoothness, and thin film-forming property has received attention as the solder resist. Such a dry film-type solder resist can impart the merits of simplification of a resist-forming process, or a reduction of solvent displacement in the resist-forming process, in addition to said characteristics.
In earlier days, a photo-curable and thermo-curable resin composition including a photo-polymerizable monomer such as a polyfunctional acrylate in company with an acid-modified oligomer, a photo-initiator, and a thermo-curable binder has been used for forming the solder resist. However, the solder resist formed from such a resin composition does not show a high glass transition temperature or sufficient heat-resistance reliability. Thus, it has disadvantages of not satisfying the PCT tolerance, TCT heat resistance, HAST tolerance to fine wiring, and the like that are required for the package board materials of semiconductor devices.